1. Field of the Invention
The present invention relates to an AC generator control apparatus for use in vehicles which serves to suppress any sudden reduction in output of the generator when the overheating safeguard function of the generator control apparatus is actuated.
2. Description of Related Background Art
FIG. 1 is a circuit diagram of a conventional AC generator for use in vehicles. In FIG. 1, an AC generator 1 comprises an armature 101 and a field coil 102. AC voltage generated by the armature 101 is applied to a rectifier 2.
The rectifier 2 comprises a full wave rectifier composed of diodes and has a main output terminal 201, an auxiliary output terminal 202 and a ground terminal 203.
The main output terminal 201 is connected to the positive pole of a storage battery 5, the negative pole thereof being connected to the ground. Electric load 6 of a vehicle is connected in parallel with the battery 5.
The positive pole of the battery 5 is also connected to the auxiliary output terminal 202 of the rectifier 2 through a key switch 7 and an initial exciting resistor 8 and is connected to a collector of a power transistor 305 of a voltage regulator 3 through the field coil 102.
The auxiliary output terminal 202 of the rectifier 2 is connected to the ground through voltage dividing resistors 301 and 302 for purposes of voltage detection. A node between the resistors 301 and 302 is connected to a base of a controlling transistor 304 through a Zener diode 303 and diode 308 in order to prevent reverse currents.
An emitter of the transistor 304 is connected to the ground and a collector thereof is connected to a base of the power transistor 305. An emitter of the power transistor 305 is connected to the ground, the collector thereof being connected to the auxiliary output terminal 202 of the rectifier 2 through a suppression diode 307, and the base thereof being connected to the terminal 202 of the rectifier 2 through a resistor 306.
The voltage regulator 3 is composed of the voltage dividing resistors 301 and 302 for voltage detection, Zener diode 303, controlling transistor 304, power transistor 305, resistor 306, suppression diode 307, and reverse current preventing diode 308.
Reference numeral 4 denotes an excessive temperature controller. A series circuit of a resistor 401 and a Zener diode 402 is connected between the ground and node between the key switch 7 and resistor 8. Point A between the resistor 401 and Zener diode 402 serves as a constant voltage source which supplies voltage to excessive temperature controller 4.
Reference numeral 410 denotes a resistor and 411 is a temperature detecting sensor and these are serially connected between the power source and the ground. The sensor 411 detects the temperature of the AC generator 1.
Point B between the sensor 411 and resistor 410 represents a detection voltage output node of the sensor 411. The detection voltage output node B is connected to a negative input terminal of a comparator 407.
A positive input terminal of the comparator 407 is connected to a node between voltage dividing resistors 408 and 409 for supplying a reference voltage. The resistors 408 and 409 are serially connected between the constant voltage source and the ground.
An output terminal of the comparator 407 is connected to a rectangular pulse oscillator 403 through a diode 404 in order to prevent reverse currents. The output terminal of the comparator 407 is also connected to the constant voltage source through a resistor 406 and connected to the base of the transistor 304 in the voltage regulator 3 through a diode 405 in order to prevent reverse current.
The operation will now be described. First, by turning on the key switch 7, a field current flows from the battery 5 to the field coil 102 of the AC generator 1 through the key switch 7 and initial exciting resistor 8, whereby the AC generator 1 starts the power generating operation.
The AC voltage generated by the AC generator 1 appears in the armature coil 101 and the AC voltage is rectified by the rectifier 2. The rectified voltage appears at the main output terminal 201 and auxiliary output terminal 202.
The rectified voltage at the main output terminal 201 is applied to the storage battery 5 and electric load 6 and the battery 5 starts charging.
The rectified voltage at the auxiliary output terminal 202 or the voltage of the battery 5 is applied to the voltage dividing resistors 301 and 302 of the voltage regulator 3. When the voltage divided by the resistors 301 and 302 rises above a predetermined voltage level, that is, a Zener voltage of the Zener diode 303, the Zener diode 303 is made conductive.
In this way the transistor 304 is turned on and the power transistor 305 is turned off. Thus, the field current of the field coil 102 is shut off and the output voltage of the AC generator 1 may decrease.
When the output voltage of the AC generator 1 or the voltage of the storage battery 5 drops below the predetermined value, the Zener diode 303 is turned off and the controlling transistor 304 is also turned off. The power transistor 305 is then turned on and a field current flows through the field coil 102. Thus the output voltage of the AC generator 1 may increase, whereupon similar operations are repeated in the above manner. The output voltage of the AC generator 1 is controlled by the voltage regulator 3 so that a predetermined constant voltage is held at a substantially consistent level.
The operation of the excessive temperature controller 4 will now be described. FIG. 2 is a characteristic graph showing the relationship between the output of the AC generator 1 and the temperature of the generator 1. When the temperature detecting sensor 411 detects that the temperature of the AC generator 1 exceeds an allowable temperature due to an increase in temperature of the vehicle and reaches an upper limit temperature T.sub.2 indicative of the overheated state, the voltage at the detection voltage node B drops below a reference voltage which is determined by the voltage dividing resistors 408 and 409.
The output of the comparator 407 therefore becomes high level. The power transistor 305 of the voltage regulator 3 is intermittently turned on or off in accordance with the oscillating operation of the constant duty ratio oscillator 403. The output voltage of the AC generator 1 is suppressed, thereby inhibiting any rise in temperature of the AC generator 1. In this way the AC generator 1 is protected against damage or breakage due to overheating.
Since control apparatus of conventional AC generators used in vehicles are constructed in the manner mentioned above, when the temperature of the AC generator exceeds the overheating limit temperature T.sub.2, the on-time ratio (duty ratio) of the power transistor 305 suddenly decreases to the on-time ratio determined by the rectangular pulse oscillator 403. Therefore, the output voltage of the AC generator 1 also changes suddenly and this leads to a sudden change in the brightness of the head lamps or the like which is discomforting to the driver of the vehicle.
On the other hand, the driving torque of the AC generator 1 also rapidly changes in proportion to the output of the AC generator 1. This leads to the problem that the rotational speed of the engine (not shown) necessary to drive the AC generator 1 changes, and this is also unsettling for the driver.